Usage: fastfetch --color <color>

<color> must be a color encoding as ANSI escape sequences. It is inserted between "ESC[" and "m".
Infos about them can be found here: https://en.wikipedia.org/wiki/ANSI_escape_code#SGR_(Select_Graphic_Rendition)_parameters.
Examples:
   --color 35:    sets the color to magenta. `--color magenta` is also supported
   --color 4;92:  sets the color to bright Green with underline
   --color 5;104: blinking text on a blue background
If no color is set, the main color of the logo will be used.
